Process for preparing polyamides from cyclic amides



Patented May 6, 1941 raooass Fon PREPARING POLYAMIDES mom orcuc ampsWilliam E. Hani'ord, Wilmington, DeL, assignor to T E. I. du Pont deNemours a Company, Wilmington, Del., a corporation Delaware No Drawing.Application February 9, 1939, Serial No. 255,507

Claims.

This invention relates to polymeric materials and more particularly tothe preparation of polyamides. v

This case is a continuation-in-part of my application Serial Number232,682 filed September This invention has as an object a new method forthe manufacture of polyamides. A further object is a new and improvedprocess for the production oi fiber-forming polyamides of the kindheretofore obtained by polymerization of amino acids as described inPatent 2,071,253. A still further object is a process for obtainingpolyamides of. this kind which yields products having good dye amnityand resistance to discoloration or alteration upon heating. Otherobjects will appear hereinafter.

These objects are accomplished by heating epsilon-caprolactram, or othercyclic amide containing more than 6 annular atoms, under the conditionshereinafter pointed out, with a substantial amount of water.

I have found that cyclic amides oi the kind mentioned above, whentreated by the process described herein, with amounts of water not lessthan one-tenth mol per mol oi cyclic amide are rapidly converted in highyield to polyamides. In this amount water alone brings about theconversion and it is not necessary that it be used as a promoter forvarious. catalytic bodies, the presence of which is disadvantageous inthat they introduce ditllcultly removable foreign material and produceundesirable dark-colored products. With water in the amount indicatedthe conversion is markedly more rapid than it is in most instances whencatalytic bodies, either with or without a trace of water upon whichtheir efllcacy in many instances depends, are used.

In the best method of carrying out my invention the production of thepolyamide involves two steps, the first of which consists in heating thelactam and water to a temperature of from 180-300 C. undersuperatmospheric pressure (usually from 200 to 280 pounds per squareinch) to eflect partial polymerization. When most of the monomer hasbeen converted to a low polymer, the water is allowed to distillgradually irom the reaction mixture whilepolymerization continues. Thepressure is finally reduced to atmospheric, and the second stepcomprises completing the polymerization by heating at atmosphericpressure in the range of 180-300 C. It is sometimes advantageous, thoughin no wise essential, to carry out the flnal stage of the reaction underdiminished pressure to effect removal of unchanged monomer and morecomplete 01 water from the polymer.

The reaction may advantageously be carried out in the presence of asmall quantity of a viscosity stabilizer, i. e. a compound capable ofreremoval acting with an amino group or a carboxyl group 1 or with bothto form a stable derivative. This derivative is considered stable in thesense that an amino or carboxyl group which is so blocked is no longercapable of participating in the polymerization reaction. The purposes oisuch a compound are (1) to limit the length of the polyamide chains,thus providing a polymer of desired physical properties which will notundergo further polymerization or depolymerization with consequentviscosity change on subsequent heat treatment; (2) to stabilize thepolymer against "discoloration on heating in air; and (3) to impartenhanced dye amnity to the resulting polymer. Compounds suitable forthis purpose include such substances as monofunctional acids and amines,polyiunctional acids and amines, and salts oi such acids. Thesestabilizers serve no apparent catalytic purpose in promoting the poly-'merization reaction when used according to the process of thisinvention; the reaction takes place rapidly and smoothly when wateralone is prwent. Advantage is derived from the use of such stabilizers,however, in the improved properties of the resultingpolymer.

With regard to the production of light-colored:

polyamides, I have observed that when caprolactam is prepared byrearrangement of cyclohexanone oxime, some 0! the oxime is invariablypresent in the crude lactam and the actual amount varies with theconditions of the rearrangement. I have discovered that the cream tobrown color of the polyamides obtained from caprolactam converted topolymers in the substantial absence oioxygen is due to the cyclohexanoneoxime contained in the lactam, and that for the production of toughwhite polymers the oxime content of the lactam must be reduced to lessthan 0.5% and preferably less than 0.1%. Lactam of less .than 0.1%impurity dissolves in a small amount of water to a clear solution. 0nthe other hand, lactam containing larger amounts of oxime gives a turbidsolution in a small quantity of water.

Special precautions are necessary to remove cyclohexanone oxime from thelactam. Fractional distillation through a'long Fenske column packed withglass helices may be employed to accomplish separation of the oxime. Ithas been found, however, that in some cases thi is not entirelysatisfactory because of the large foreshot, boiling at practically thesame temperature as the pure lactam, which contains suflicient oxime togive a cloudy solution in water. More satisfactory is the practice ofwashing a solution of the crude lactam in an organic solvent with anaqueous saturated salt solution containing alkali. The alkali dissolvesmost of the oxime, while the saturated salt solution takes up but littlelactam. This procedure reduces .the oxime content to such a point thatcareful fractional distillation through a good column serves efficientlyto separate what little oxime remains. Epsilon-caprolactam which hasbeen thus purified can be readily converted to white, tough polymers.

The following examples are further illustrative of the methods used inpracticing my invention. Temperatures are expressed in degreescentigrade, and parts are by weight.

Example I Epsilon-caprolactam and molecular equivalent of water wereheated at 250 for 6 hours in a sealed tube with an inert atmosphere. Thetube was then opened and heated at 255 under atmospheric pressure toallow the water to distill oil and to complete the polymerization. After2 hours a high degree of polymerization had been attained as evidencedby a melt viscosity of 20301 at 255. The reaction was stopped after 2hours more, and the polymer was obtained as a very hard and tough whitesolid.

Example II A metal bomb was charged with 30 parts of epsilon-caprolactamand 19.1 parts of water (4 molecular equivalents) containing 0.063 partsof acetic acid ,4 molecular equivalent) to serve as a stabilizer. Thebomb was sealed, provided with an inert atmosphere, and heated at 250for 1.5 hours. During this period, a little water vapor was bled oil tokeep the pressure from exceeding 250 pounds per square inch. At the endof this time, the pressure was allowed to fall to atmospheric and theremoval of water and com pletion of the polymerization was effected byheating'for 1 hour at 250 under atmospheric pressure. There was obtaineda hard, tough polymer intrinsic viscosity of which was 0.70.

Example III Five parts of the mixture of isomeric lactams obtained bythe rearrangement of a mixture of Example IV Two parts of cyclo-octanoneisoxime was heated with 1 molecular equivalent of water in a sealed tubeand inert atmosphere for three hours at 250. It was then heated 1 hourunder 5 mm. pressure at 255. There was obtained a white, tough polymermelting at 178 it had an intrinsic viscosity of 0.84.

Example V A mixture of 2.5 parts of 3-methyl cyclohexanone isoxime, 7.5parts of monomeric hexamethylene diammonium adipate, and 0.2 part ofwater (This, in addition to the water liberated during thepolymerization of the hexamethylene diammonium adipate, amounts to 1molecular equivalent of water based on the isoxime), was heated in asealed tube and inert atmosphere for 4 hours at 250. Although the melthad become viscous at this time, heating was continued for an additional20 hours. The tube was then opened and the mixture heated for 3 hours at255' while a stream of moist nitrogen was passed through the melt, andthen for 1 hour with a current of dry nitrogen passing through the melt.The polymer so obtained was a tough, white solid which softened at 194and had an intrinsic viscosity of 0.85.

The quantity of water used in carrying out the present process ispreferably from one to four molecular equivalents of water and isgenerally less than ten moles of water per mole of cyclic amide. Greateramounts of water are of no advantage, and since they require greaterheat inputs and more voluminous bleed-offs, are uneconomical. When lessthan one-tenth molecular equivalent of water is used, the time requiredto obtain high molecular weight polymers becomes so long as to make theprocess impracticable.

The time of reaction is dependent on both the temperature employed andthe amount of water used. However, it is not diificult to achieve a highdegree of polymerization in 8 hours by the process of this invention.For example, a high molecular weight polymer can be prepared in 8 hoursby heating epsilon-caprolactarn with 1..- molecular equivalent of waterat 250 C. When larger amounts of 'water are employed this time may bematerially shortened, as for instance, in Example II in which 4 mols ofwater require 2.5 hours at 250 C. 1

In general I prefer to operate in the range 200-280 C., inasmuch as toolow temperatures require protracted periods of heating, and too hightemperatures tend to occasion extensive decomposition anddepolymerization. The final stage of the polymerization may be carriedout under diminished pressure in order to remove unreacted monomer andto remove the water more completely. The use of diminished pressure isnot essential to the process, however, and high molecular weightpolymers can be obtained without such treatment.

The process of this invention is, in general, applicable to monomericcyclic amides containing more than 6 annular atoms, such ascyclohexanone isoxime (epsilon-caprolactam) the methyl cyclohexanoneisoximes, cycloheptanone isoxime, cyclo-octanone isoxime,cyclopentadeoanone isoxime, cyclohexadecanone isoxime and monomericcyclic hexamethylene adipamide; it is particularly applicable, however,to epsiloncaprolactam. These isoximes may be obtained by therearrangement of the corresponding cyclic ketoximes with sulfuric acid.The cyclic amides used are preferably those which are not heavilysubstituted since certain lactums which bear several substituents on thering, such as the lactams obtained by rearrangement of menthone oxime,resemble 5- and fi-membered rings in hav ing a marked tendency towardring closure as opposed to intermolecular reaction once the ring isopened.

Substances which may serve as stabilizers for polymers produced by theprocess of this invention include monobasic acids, such as acetic,propionic, butyric, and stearic acids; dibasic acids, such as adipic andsebacic acids; ammonia; monoamines, such as methylamine, isobutylamine,amylamine, and stearylamine; diamines, such as hexamethylene diamine anddecamethylenediamine; salts of acids, such as sodium acetate andammonium chloride; amino acid derivatives, such asepsilon-benzoylaminocaproic acid and epsilon -(p-toluenesulfonylamino)-caproic acid; and amino alcohols, such as ethanolamine. The quantity ofstabilizer used is dependent on the properties desired in the resultingpolymer. In general, the stabilizer is added in the proportion of from 1to ,5 molecular equivalents based on the lactam, if a fiber-formingpolymer is desired.

The process of this invention is also applicable to the production ofinterpolymers, such as interpolymers prepared from lactams and aminoacids, hydroxy acids, diamine-dibasic acid, glycol diester, and glycoldiacid combinations. That is, it may be used in the production ofinterpolymers from lactams and other polymer-forming reactants.

If a colored product is desirable, the process of this invention may becarried out in the presence of a pigment. The polymers may also bemodified by carrying out the polymerization in the presence of aplasticizer or other modifying agent.

The polymers formed by the process of this invention, which areapparently chemically identical with those obtained fromepsilon-aminocaproic acid, are characterized by their high meltingpoints (212-214 C.) and by the fact that they can be formed intofilaments which yield oriented fibers on the application of tensilestress in the solid state, e. g. cold drawing. They are furthercharacterized by their microcrystalline nature, as evidenced by theirsharp melting points and X-ray diffraction patterns. In general, toobtain polymers capable of being formed into oriented fibers, theheating should be prolonged until the polymer has an intrinsic viscosityof about 0.4. Intrinsic viscosity is here used as defined in Patent2,130,948.

The polyamides obtained by the practice of this invention, in additionto being useful in the manufacture of fibers, are useful in theproduction of films, coating compositions, and molding compositions.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that I do not limit myself to the specific embodimentsthereof except as defined in the appended claims.

I claim:

1. A process for preparing polyamides which comprises heating a cyclicamide as an initial polyamide,-forming reactant under superatmosphericpressure with water in amount of at least 0.1 mol of water per mol ofthe cyclic amide, and continuing said heating under superatmosphericpressure with retention of said amount of water until the cyclic amidehas been converted mostly to polyamide, said cyclic amide containingmore than 6 annular atoms in which the nitrogen is amido nitrogen and isan integral part of the IIIlg.

2. The process set forth in claim 1 in which said cyclic amide is heatedin a closed system in the presence of from 0.1 to 10 mols of water permol of lactam.

3. A process for preparing polyamides which comprises heating a cyclicamide as an initial polyamide-forming reactant at 180 to 300 C. undersuperatmospheric pressure with from 0.1 to 10 mols of water per mol ofthe cyclic amide and continuing said heating under superatmosphericpressure with retention of said amount of water until the cyclic amidehas been converted mostly to polyamide, and then removing water from thereaction and continuing polymerization at said temperature with removalof water until a high molecular weight fiber-forming polyamide isobtained, said cyclic amide containing more than 6 annular atoms, inwhich the nitrogen is amido nitrogen and is an integral part of thering.

4. The process set forth in claim 1 in which said cyclic amide isepsilon-caprolactam.

5. The process set forth in claim 1 in which the water is present inamount of from 1 to 4 mols of water per mol of the cyclic amide.

6. The process set forth in claim 1 in which said amide isepsilon-caprolactam and in which the water is present in amount of from1 to 4 mols of water per mol of the cyclic amide.

7. The process set forth in claim 3 in which said cyclic amide isepsilon-caprolactam.

8. The process setforth in claim 3 in which the water is present inamount of from 1.to 4 mols of water per mol of the cyclic amide.

9. The process set forth in claim 3 in which said amide isepsilon-caprolactam and in which the water is present in amount of from1 to 4 mols of water per mol of the cyclic amide.

10. A process for obtaining light colored polyamides fromepsilon-caprolactam containing cyclohexanone oxime which comprises thestep of reducing the oxime content to less than 0.5% and heating thepurified lactam under superatmospheric pressure with water in amount ofat least 0.1 mol of water per mol of the lactam and continuing saidheating under superatmospheric pressure with retention of said amount ofwater untila polymeric product is obtained.

WILLIAM E. HANFORD.

